What is the black cap on the tube coming out of the transmission called and what is it for?

[casmer]

There are two black tubes coming up out of the transmission boxes on my craftsman ZTS 7500, what are they for and what are those caps called? looking for a part number if anyone has it as one of mine is missing, and my guess is these should be close up to keep dirt out.

 

[StarTech]

What you have in your hand is the vent cap.

Need the model off the transaxle which is likely a Hydro Gear as that part is provided by them and not Craftsman. And for the Craftsman ztr ZTS 7500 we would need the model number off the serial tag which usually start 917 if any other parts are needed..